This invention relates to application of a coupling onto the end of a threaded pipe and, more particularly, to an improved method and apparatus for automatically locating critical coupling parameters and for real-time monitoring of the coupling application.
Pipe sections used in the oil industry for line pipe, well casing, etc. usually have tapered exteriorly threaded ends. These pipe are joined together using couplings in the form of collars which have interior tapered threads extending inwardly from each end. In order to form a leak tight joint between pipe sections, a thread compound is applied to the coupling threads, the coupling is threaded onto one end of a pipe, and a second pipe is threaded into the coupling. Since the pipes must be joined in the field, the pipes are usually ordered from manufacturers with a coupling attached to one end. Due to the high internal pressures which such pipe must withstand in service, each pipe and coupling is manufactured and inspected to rigorous standards and each coupling must be installed according to rigid industry and customer specifications.
There are two major factors which determine whether or not a coupling has been applied to a pipe in an acceptable manner. The first is the length of engagement of the threads of the coupling with those of the pipe. The second is the amount of torque required to screw-on the coupling to the specified final position. The thread dimensions, final coupling position and other coupling parameters are the subject of specifications issued by the American Petroleum Institute ("API") for the various pipe types and sizes.
API's Specification for Threading, Gaging, and Thread Inspection of Casing, Tubing and Line Pipe Threads, API Specification STD 5B, 13th Edition, May 31, 1988, sets forth the required basic dimensions of pipe and coupling threads, as well as hand-tight and power-tight positions on page 7, FIG. 2.1. One such dimension, L.sub.4 is the measurement of the total length of the threaded portion of each end of the pipe from the end of the pipe to the point that the threads vanish. This length varies according to the pipe diameter. Each coupling is required to have a particular length of threads from the applied bearing face for proper mating with the pipe threads. This coupling thread length also changes with pipe diameter and thread type.
As observable in FIG. 2.1 of API STD 5B, a 3/8 inch high equilateral triangle is die-stamped onto the pipe surface to indicate the desired final position of the applied face of the coupling after power tightening. The location of this triangle stamp depends on the thread type. For buttress thread, API requires the triangle to be at L.sub.4 plus 0.300 inches for pipe sizes 133/8 inches in diameter and smaller and at L.sub.4 plus 0.200 inches for pipe sizes 16 inches in diameter and larger. For 16, 185/8 and 20 inch diameter 8-round thread casing in Grades H40, J55 and K55, API requires the triangle to be at L.sub.4 plus 1/16 inch (0.0625 inch). Although API does not require a triangle stamp for some pipe sizes and thread types, the preferred standard practice is to place a triangle stamp at L.sub.4 plus 1/16 inch for all pipe except buttress, with buttress stamped at L.sub.4 plus 0.200/0.300 inches as indicated above.
A coupling is usually installed to one end of each pipe in a two step process as follows. The coupling is threaded on to the "hand-tight" position, which corresponds to the position at which the threads of the pipe and the coupling are engaged to such an extent that any further threading would cause interference between the pipe and coupling threads. Since the final desired position, known as the "power-tight" position, of the coupling face is known and the thread pitch is known, the number of turns of the coupling required to reach the final position from the hand-tight position can be determined. While the pipe is held stationary, the coupling is rotated the number of turns so determined and the torque being applied is measured. If a specified maximum torque value is reached before the coupling reaches the desired final position, the pipe is rejected. Also, should the coupling be screwed on to the final position without reaching a minimum torque value, the pipe is rejected. Therefore, there is an allowable window of make-up torque for acceptability of each pipe and coupling.
In order to confirm that a pipe or coupling is joined in an acceptable manner, it is critical to determine the actual hand-tight position or reference point as the coupling is being screwed on. In some prior art coupling systems, a pre-determined value is assigned as the reference torque and the hand-tight position is presumed to be the position of the coupling when the level of applied torque reaches this level. Whenever an irregularity in the pipe or coupling threads or in the state of out-of-roundness of the pipe or the coupling causes a spike in the torque measurement which exceeds the reference torque value, this system provides an erroneous reading of the hand-tight position, resulting in a failure to accurately determine the final make-up conditions of the pipe joint.
In the present invention, a computer analysis of a combination of known pipe and coupling dimensions, position measurements obtained through image analysis, counting of turns of coupling rotation and torque measurements is used to determine the precise hand-tight position and to map the turns and torque values during rotation of the coupling from the hand-tight position to the final power-tight position. In this manner, a coupling can be automatically installed on pipe to the required specifications and the make-up parameter data stored for analysis.